While sausages cool, discard all but 1 tablespoon drippings from Dutch oven. Add oil to Dutch oven and heat on medium. (If your sausages are very lean and you do not get 1 tablespoon drippings, add enough additional oil to drippings to equal 1/4 cup fat total.) Gradually stir flour into drippings until blended, and cook 4 to 5 minutes or until flour mixture (roux) is deep brown, stirring constantly. Add green pepper, onion, celery, and garlic, and cook 5 to 6 minutes or until all vegetables are tender, stirring occasionally.